1. Field of the Invention
This invention relates to solar power generation, and more particularly to multicolor solar power generation means.
2. Description of the Prior Art
A large body of work has been done on the subject of photocell materials. Considerable work has been done on the subject of optimum sunlight concentrating geometries. There has been speculation regarding constructing photovoltaic solar power systems using an assembly of dichroic filters and various photocell materials to achieve greater integrated conversion efficiency by taking advantage of the high efficiency of particular photocell materials in relatively small spectral ranges. An article summarizing the current state of the art appeared in the IBM Journal of Research & Development, Vol. 22, No. 2, March, 1978, entitled, "Novel Materials and Devices for Sunlight Concentrating Systems."
While the multicolor system described in the above article is expected to achieve greatly improved conversion efficiency, it suffers from certain inherent limitations, notably bulkiness, excessive weight, and consequent high cost.
U.S. Pat No. 4,021,267 discloses a system which has the same goal as the present invention, but accomplishes the goal (i.e., spectral separation of sunlight concentrated onto a photocell array) by an inherently less efficient and more complex, massive, cumbersome, and expensive method than is disclosed herein. U.S. Pat. No. 4,021,267 is based upon the reasonable but erroneous conclusion that "If an attempt is made to use the system with a concentrator without some form of collimation, the spectral separator will not efficiently separate all the light since the incident radiation to the spectral separator does not consist of parallel beams. As a result, a substantial portion of the light passing through the separator will still be "white." Thus, while in the inventor's system it is still necessary that the area of the photovoltaic cells be greater than the area of the spectral separator upon which the incident radiation impinges, the use of concentrator and collimating elements permits much higher intensity radiation to be passed to the photovoltaic cells through the spectral separator." That conclusion leads to his design which requires a three-element optical system: a concentrator, a collimator and a spectral separator.
There are several important disadvantages to the above system. First, it necessitates the use of three separate optical elements, each of which will have attendant and unavoidable optical losses with consequent reduction of overall collection efficiency. With such losses reasonably amounting to 20 or 30 percent in each optical element, overall loss can easily amount to more than 50 percent, with the losses thus offsetting the gain in photocell efficiency due to spectral separation.
Second, the degree of concentration and spectral separation attainable in the system is not optimum. Optimum concentration and spectral separation can only occur where the image of the sun is in focus on the photocells for any particular spectral component; yet the system described does not focus the sun's image on the photocells.
Third, with a three-element concentrating and separating system, the system is bulkier and heavier than is desirable.
The present invention accomplishes the desired task of optimum concentration and spectral dispersion with a single, very inexpensive optical element, thereby avoiding the need for additional optical elements, the attendant unavoidable light losses, and the additional structures required to support the additional optical elements.
While the concept of using spectrally separated light to obtain higher efficiencies in a photovoltaic generating system is known, the single-element spectrally dispersive collector disclosed herein and the generating systems constructed therefrom are entirely novel and represent significant advances in the art of solar power generation. Reflective, focussing diffraction gratings have been used in the past in spectrometers, but their use as concentrators and spectral separators of sunlight is entirely novel.